Crucially, Mallary’s work doesn’t violate the weak energy condition. The objects do not need to have negative mass. It does, however, require the objects to contain singularities—which as far as we know are found in black holes—with infinite density.

“Moreover, unlike singularities that are present in the interior of black holes, which makes them totally inaccessible from the outside, the singularities in Mallary’s model are completely bare and observable, and therefore have true physical effects,” Khanna said. “Physicists don’t expect such peculiar objects to exist in nature, either.”

This is all highly theoretical. And the solution is, for now, as impossible as the original problem. But at least on paper, one reason why time travel is not possible may have been intellectually dispelled.

📬 Sign up for the Daily Brief

Our free, fast, and fun briefing on the global economy, delivered every weekday morning.